2 Answers

I found this on Quora by Clark Eller.  

Why Aren't Airplane Black Boxes Designed To Be Bouyant?

 If black boxes floated, they would be much more likely to drift away never to be found.

The placement of the box in the tail of the aircraft, one of the most structurally strong areas of the plane, and a section that usually remains more or less intact in crashes, provides some additional protection and makes it easier to retrieve.

If it were floating with the tides and currents who knows where it would end up.  Maybe a desolate beach washed up on shore or perhaps found by a fisherman?  Way too many variables to consider and lots of risks it will be lost.  Improving the length of time the pinger remains active would help.  Or maybe on a crash it makes calls to a satellite that list its coordinates.  Maybe in a water landing it shoots off a little floating transmitter like what happens when whales, Sharks and other things are tagged and eventually a small emitter surfaces and begins transmitting data to the waiting satellite.  Lots of things to consider but the floating I'm sure has been considered and ruled out for a number of reasons.

https://www.quora.com/Why-arent-airplane-black-boxes-designed-to-be-buoyant

Wow. I had hoped the plane would be found kidnapped and all on board safe and sound.  A ping? You’re right, it should have been designed to float……...